This ignores the question of who the lefties are, but many managers don't want to start lefties back to back against the same team to get the most out of their pitchers.

It's mostly about percentages, and analyzing baseball by percentages is a simplistic thing even if higher math is involved. Managers will relieve lefties in favor of righties to face Jose Abreu, even though Abreu hits righties better than lefties, because they are playing global percentages.

Most managers are afraid to go against the "book" to the point of extreme examples like the one above. If a manager had a lefty to face Abreu who then took him deep, that manager would be excoriated. Some would call for his immediate firing. Even if the lefty had previous success against Abreu. Guys like Casey Stengel and Leo Durocher wouldn't get much of a chance in today's game.